In the packaging industry, it is conventional that a product is packed in a plastic film or a rigid plastic package which is then heat sealed. The film forming the package usually comprises a substrate layer and a sealant layer. The sealant layer is the topmost layer of the film and is in direct contact with the packed good. The seal must possess a mechanical resistance sufficient to maintain the tight-seal properties during storage and transport. This seal strength is usually measured in Newton per 15 millimeter (N/15 mm) and corresponds to the necessary force to open a sealed film of 15 mm width. Usually, the seal strength increases with the sealing temperature up to a temperature above which the seal cannot be opened without destroying the film.
For applications related to peelable seal films and packages, the seal strength must be weak enough to allow an easy opening of the film and package by the consumer. The seal strength of peelable seal films and packages should stay below 10 N/15 mm over a wide temperature range, that is between the seal initiation temperature (as defined here below) and about 160° C., or even about 170° C. (the so called “Peel Range”). This enables a certain degree of freedom in setting the seal temperature at the sealing machine during the commercial manufacture of films and packaging which eventually have to have a peelable seal behavior.
Furthermore, in industrial applications, it is desired that these packaging films be suitable for high speed machine packaging. In this view, it is useful to provide a sealant layer that can be sealed at relatively low temperatures in order to allow for higher packaging line speeds of packaging machines. The minimal temperature allowing for sufficient seal strength in a certain application (this sufficient strength can be set at 1 N/15 mm for instance) is called the seal initiation temperature (SIT). It would therefore be useful to have sealant layers showing a relatively low heat seal initiation temperature.
Also, it is observed that after the heat sealing stage, the seal strength of the packaging increases with cooling. Usually, a great percentage of the final seal strength is obtained after several milliseconds. However, because of the high speed machines used on packaging lines, it is necessary to reach an appropriate seal strength right after the heat sealing stage so that the packaging does not delaminate on the line before it is cold. This property is called the hot tack. It would therefore be useful to have sealant layers showing a good hot tack.
In many cases the packed goods are food such as meat, pet food and dairy products, or other perishable goods such as medical and pharmaceutical products. In order to increase the shelf life of such goods, that means the time span within which they can be safely consumed without concerns regarding bacteria proliferation, the packed good, that is the package containing the good, undergoes retorting treatment so that the shelf life under non-refrigerated conditions is extended.
Retorting treatments usually consist in heating the entire package up to temperatures between about 100 and about 140° C. during a period of time ranging between about 5 and about 30 minutes. An external counter pressure is optionally applied during the process in order to avoid the deformation of the package due to the thermal expansion of the gases contained therein.
It is therefore evident that the entire package, that is the food and in particular the sealant layer of the packaging multilayer film, has to be resistant to this kind of treatment. Hence, the polymer composition forming the seal layer should have a low SIT but at the same time a sufficiently high melting temperature to undergo retorting conditions.
U.S. Pat. No. 5,272,210 describes a film made of a mixture of a propylene-ethylene copolymer and an ethylene-alkyl acrylate copolymer, with a minimum amount of alkyl acrylate in the mixture, showing good RF sealing properties.
However, there is still a need for retortable, sealant layers for packaging films that would have a relatively low heat seal initiation temperature, a good heat seal strength, and a good hot tack.